This invention generally relates to the art of electrical cables and, particularly, to flexible printed circuitry or cable (xe2x80x9cFPCxe2x80x9d). Specifically, the invention relates to grounded flexible printed circuitry wherein a grounding grid provides improved impedance characteristics for high frequency signals transmitted by the circuitry.
Electrical circuitry often is provided with protection from electromagnetic interference (EMI) and radio frequency interference (RFI) emanating from or entering the system. Although EMI and RFI now are often used interchangeably, EMI has been used to connote energy occurring anywhere in the electromagnetic spectrum and RFI has been limited to interference in the radio communication band. EMI energy can be generated outside as well as inside the system. External EMI energy can interfere with the operation of electronic equipment within the system, while internal EMI energy can create xe2x80x9ccross talkxe2x80x9d and xe2x80x9cnoisexe2x80x9d which can cause erroneous data transmission.
Electrical connectors are particularly prone to disruptions from EMI energy because of the numerous contact areas and openings for electrical terminals and cables, but numerous electrical connectors have been designed with shielding that is effective against EMI/RFI energy. On the other hand, it often is desirable to have a shielded cable as well as shielded connectors.
Conventional flexible printed circuitry in the form of a cable includes a flexible insulating base or substrate having a plurality of signal conductors extending longitudinally along one or both sides of the substrate. If it is desired to shield the flexible printed circuitry, the signal conductors typically are disposed on one side of the substrate and a ground circuit is disposed on the opposite side of the substrate. For instance, it is known to cover the opposite side of the substrate with a layer of metal such as copper for shielding and grounding purposes. However, it has been found that such metal layers significantly decrease the flexibility of the flexible printed circuitry. Consequently, a conductive wire mesh or grounding grid has been used on a side of the substrate opposite the signal conductors for grounding and shielding purposes. Such grounding grids are formed by a plurality of straight ground conductors which crisscross each other to define symmetrical open spaces therebetween.
One of the problems with the prior art grounding grids for use in flexible printed circuitry is that they do not provide a substantially constant impedance for the signal conductors extending longitudinally of the cable. More specifically, the grounding grid is formed by ground conductors which are printed or otherwise deposited on one side of the flexible insulating substrate of the cable. The ground conductors are in the form of a screen or grid with a plurality of generally straight, parallel ground conductors intersecting other parallel ground conductors at regular intervals to define equal-in-shape and equal-in-size open spaces between the conductor wires in a regular geometric pattern. Because of the regularity of the pattern, the signal conductors traverse different conductor sequences running longitudinally of the cable at different lateral positions on the cable, thus causing each conductor to have a different impedance. The characteristic impedance of flexible printed circuitry can affect the integrity of the signals travelling in the cable, particularly for high frequency signals. The creation of different conductor impedance by regular-patterned grounding grids is explained more graphically in the Specification in relation to FIGS. 3 and 4.
The present invention is directed to solving these problems by use of a grounding grid having a novel random geometric pattern.
An object, therefore, of the invention is to provide a new flexible printed circuitry having improved impedance characteristics.
In the exemplary embodiment of the invention, the flexible printed circuitry includes an elongated flexible insulating substrate. A plurality of signal conductors extend longitudinally along at least one side of the insulating substrate. A grounding grid is disposed on the insulating substrate. The grounding grid has a random or substantially random geometric pattern to provide substantially constant impedance for the signal conductors lengthwise of the FPC.
As disclosed herein, the signal conductors are disposed on one side of the insulating substrate, and the grounding grid is disposed on an opposite side of the substrate. The signal conductors extend generally parallel to each other longitudinally of the substrate. The grounding grid covers substantially the entirety of the substrate.
The grounding grid is formed by a plurality of intersecting ground conductors defining open spaces therebetween. According to the invention, the intersections of the ground conductors are in a random pattern to define unequal-in-shape and unequal-in-size open spaces therebetween.